This invention relates to missile guidance systems, and more particularly to a high PRF or CW doppler processor using the single bit playback principle.
The prior art requires the use of a large number of similar narrow band doppler filters with the results that these systems grow linearly in both size and cost as either the required doppler coverage is increased or the doppler filter bandwidth is decreased. This invention eliminates this need for a large number of expensive narrow band filters.
The use of digital techniques enables a designer to choose filter characteristics with a degree of freedom not available with the sole use of analog devices. A missile can operate with 400 Hz doppler filters, yet target spectrums of 10 to 200 Hz are expected. By narrowing the radar processing doppler filters, predetection S/N ratios can be improved from 3 to 13 db.
The acquisition time for the missile can be reduced by matching the processor's search band to the expected uncertainty in target doppler. The missile acquisition filter band configuration consisting of five filters, limits a single look search to a 2 KHz band. Therefore, when the target acquisition uncertainty exceeds 2 KHz (as it does for all modes), the search band must be stepped in frequency until the region of uncertainty is covered. Each step requires the same illumination period so that for n steps, n illumination periods are required. By widening the bank, the number of illumination periods required for acquisition can be reduced. One of the advantages of this invention is to increase the single look search band from the present 2 KHz to 5 to 10 KHz without increasing the size or cost of the processor.
A continuous mode missile tends to be detection range limited, while a sample data missile tends to be angle tracking limited. Pause-on-target (POT) performance studies indicate that it is desirable to increase the S/N ratio over that which is available, with missile range tracking doppler filters. Narrow band doppler filters can yield at least 3 db (200 Hz filters) improvement over a 400 Hz angle tracker and approximately 10 db over a 2000 Hz tracker.
Investigation of digital processing techniques has indicated that a hybrid missile radar processor (one that employs digital storage and analog filters) is not only feasible but has significant advantages over the classical all-analog approach to processing.
Present analog processors are essentially all-analog devices that operate in real time with data which is continuous. As a result, the processor requires many distinct circuits to perform similar but simultaneous operations. For example, five acquisition circuits are required to detect a target in a 2 KHz band and determine its doppler shift within 400 Hz. If the acquisition band were extended to 4 KHz, then 10 circuits are required.
In contrast with the requirements of analog devices, digital devices are not constrained to operate in real time nor are they required to be supplied with a continuous flow of data. Furthermore, the flexibility of digital equipment enables multiple use of a single device.
The playback radar processor employs a relatively fast sampling rate, f.sub.s, (40 KHz as an example) to record the sum channel signal into a storage unit. Recording of the data continues for the duration of the required integration period .tau.. At the end of this period, the digital storage unit is read out at a rate, R, significantly higher (2 to 20 MHz) than the sampling rate.